The present invention relates to electrophotographic reproduction systems and, more particularly, to color electrophotographic reproduction systems.
Electrophotographic reproduction equipment is finding increasing use. This is particularly so for full color reproductions which can be provided with very high quality using electrophotographic methods. Such methods are used for both copiers and for very much higher resolution color proofing printers.
An example of such a system is shown in FIG. 1 in a highly schematic form. The electrophotographic process is practiced on the outer cylindrical surface of a drum, 10, that is selectively rotated by a stepper motor, 11, under the control of a control system, 12. Drum 10 is formed of a metal core, 13, which can rotate in journals supported on a frame, not shown, about a rotation axis that is essentially its axis of symmerty with respect to its cylindrical outer surface. The cylindrical outer surface portion of metal core 13 has a plastic layer, 14, as a substrate wrapped therearound. An electrically conductive surface layer, 15, is provided on plastic layer 14, and an organic photoconductor, 16, is coated on that conductive surface which is electrically connected to ground through metal core 13.
The circumference of the cylindrical surface of drum 10 has been selected to be 846.667 mm is this example. A typical surface velocity of the exposed surface of drum 10 during a reproduction cycle would be about 5 mm/sec. Stepper motor 11 has been chosen in this example to provide 200,000 steps per complete revolution of drum 10.
In the electrophotographic reproduction process, organic photoconductor 16 is charged to a surface potential of from typically 250 V to 400 V positive with respect to ground, selected portions of that surface thereafter being discharged by a modulated, scanning laser beam to a lower potential at those locations encountering sufficient beam intensity under the modulator signal to result in forming a desired electric charge pattern, or potential pattern, on that surface. This pattern is provided in accord with a color separation signal which specifies the desired locations of a constituent color in a desired resulting printed image which is typically formed of three or four such attract a selected toner having a desired constituent color, this attracted toner subsequently, being transferred from the surface of drum 10 along with other colored toners to the surface of the medium on which the printing is to occur to form a printed image.
An electrifier, 17, such as a scorotron supplies electric charge to the entire adjacent surface portion of photoconductor 16 to cause that portion to reach the above peak surface potential prior to its reaching the region of intersection with the scanning laser beam. A toning developer arrangement, 18, contains six identical units, 19, each containing an alternative one of the four constituent color liquid toners that might be each used to form a corresponding subimage enroute to forming a complete color printed image plus two other alternative color toners for any special effects desired. The four colors typically are black, cyan, magenta, and yellow liquid toners.
In each unit 19, there are pumping means to supply the toner to the surface of a moving band, 20, provided in each, this band being capable of being rotated across the outer surface of drum 10 parallel to the rotation axis thereof. A motor arrangement, 21, is controlled by control unit 12 to position a selected one of toner units 19 so that a surface of the band 20 therein engages the surface of photoconductor 16 of drum 10 resulting in the toner in that unit being attracted to this surface of photoconductor 16.
A selected toner unit 19 has its band 20 charged to a voltage from 150 V to 200 V positive with respect to ground which charges the toner pumped thereover from one side of the band. A vacuum provision arrangement is provided in each toner unit 19 on the other side of the band therein to remove excess liquid toner.
The scanning laser beam, modulated by a corresponding color separation signal to provide the associated electric charge pattern on the surface of photoconductor 16 by selectively discharging that surface, does so successively for each of toner units 19. Thus, an initial charge pattern is provided on photoconductor 16 followed by a corresponding toner deposition step, and then a new charge pattern is provided on photoconductor 16 under the previous toner or toners each time there is a completion of the deposition of the toner for a previous charge pattern until the final toner to be used has been deposited. Each of the corresponding toners attracted to its charge pattern is deposited as a subimage and accumulated on photoconductor 16 to form the complete toner image.
This complete toner image is subsequently transferred onto an intermediate medium formed by a coated polyester web, 22, which coating contains a thermally sensitive adhesive layer and a release/ protective layer. Web 22 is forced against layer 16 on drum 10 by a heated roller, 23, which results in a transfer of the accumulated toner on photoconductor 16, to web 22 through its being picked up by the adhesive layer therein. A later step results in transferring the accumulated toner, the adhesive layer and parts of the release/protective layer from web 22 onto the medium on which printing is to occur, such as paper, to thereby provide an up to six color half-tone printing result.
A laser electromagnetic radiation source arrangement, 24, under the control of control unit 12, supplies the laser beam for selectively discharging the surface of photoconductor layer 16 drum 10 which is modulated by control unit 12 using such corresponding color separation signals as are obtained from a memory, 25. Laser beam source 24 correspondingly supplies a modulated laser beam, 26, through an optical beam conditioning unit, 27, to impinge on an eight-faceted, rotating polygon mirror arrangement, 28, which is rotated by a motor, 29, again operated by control unit 12. Laser beam 26 is reflected from successive facets of rotating polygon 28 to then pass through further processing optics, 30, so as to repeatedly scan from left to right across that portion of the cylindrical surface of photoconductor 16 and drum 10 that is rotated thereunder.
This arrangement involving laser beam 26 is more clearly seen in FIG. 2. There, the scan range of laser beam 26 is shown from one of the facets of rotating polygon 28 by a pair of dashed lines, 26', this range being set by a pair of laser beam radiation detectors, 31 and 32, each formed by a mirror and photosensor arrangement in a fixed position with respect to each other and to the rotation axis of drum 10. Each of detectors 31 and 32 provide an electrical signal, corresponding to any such radiation impinging thereon, to control unit 12 to indicate when scanning laser beam 26 is at the beginning of its scan range, that is, at sensor 31, and when it is at the end of its scan range, or sensor 32.
Motor 29 can rotate polygon 28 at a selected rate between 2,000 and 4,000 revolutions per minute (rpm) with each revolution providing eight scans corresponding to the eight facets of this polygon. The rotation of polygon 28 is controlled by control unit 12 in a phase-lock loop arrangement to thereby provide sufficient angular velocity control of polygon 28 so as to have a repeatability on the order of 62 parts per million (ppm) maximum deviation over relatively long periods of time. This polygon velocity will have a deviation of only 42 ppm over periods of 20 to 30 minutes during which a sequence of color printings, each corresponding to a color separation signal, will be made to form a single printed color image.
At 4,000 rpm, rotating polygon 28 will provide a scan line every 1.875 ms. For the rotation data and size data of drum 10 given above, drum 10 will revolve once in every 166.667 seconds so that a stepping motor providing 200,000 steps per revolution will require 0.833 ms per step. Thus, drum stepping motor 11 will provide 2.25 steps per scan line giving a 4.17 .mu.m change in the position of the surface of drum 10 per step of stepping motor As a result, there is the possibility of being able to distinguish between scan lines sufficiently well so that the registration of successive color separation subimages for forming a color printed image can meet a .+-.10 .mu.m requirement if the relative positional relationship between rotating polygon 28 and rotating drum 10 can be established with sufficient accuracy.
There is a strong desire to meet such a registration requirement to provide high quality printing. Such a result is obtained if laser beam 26 can be positioned so accurately on the surface of drum 10, at the start of each color separation cycle needed to form an image, that it is never displaced from the desired initial location by more than half the spot size of that beam. The beam spot size of laser beam 26 in its longest dimension along the direction of rotation of drum 10 is typically set at 20 .mu.m to give sufficient resolution and yet maintain a reasonably cost effective optical system. Thus, there is a strong desire to start each successive charge pattern on drum 10, provided by the discharge or exposure of photoconductor 16 by laser beam 26 and corresponding to each color separation printed subimage used in forming a color printed image, at a point within .+-.10 .mu.m of the desired start location.